I've been working on a Pioneer PL-Z91 for a bit because of some pretty loud hum it produces. I've reduced the problem to the turntable: I've tried the same turntable on a different set of receiver/speakers and hum was still there, and I've tried a different turntable on the original receiver/speakers and no hum was present.

Hum tends to grow when I touch the tonearm, which leads me to think the problem might be tonearm grounding. I've attached pictures of something that might be the ground, attached to a metal plate inside the turntable.

That could and may be the tonearm ground. See that little board with the four tiny wires? Those are the cartridge wires from the tone arm. They can cause hum too when they are exposed. Usually they are covered with a grounded metal plate or perhaps a grounded woven metal tube. Tracking down hum is difficult sometimes.

Hi, that black wire with the screw is part of the ground. On the small circuit board you can see the letters GND that stands for ground. And the black wire looks to be soldered into that spot on the underside of the circuit board, you can flip it over to check. Here's what I would do I would run a length of wire from under that screw to the back of your receiver and see if that eliminates the hum. If this does the trick to eliminate the hum then you can just lever the wire there and make sure the screw is tight. Or you can solder it to the spot where the black wire is attached. I can also see that the black ground wire from the left channel from the tonearm and a yellow wire that's the ground from the tonearm are soldered together on that small circuit board. If you have a digital volt meter you can use that to check for continuity of those wire. Set the meter to 2000 ohms, then remove the cartridge. Using the probes from the meter put the end of one probe onto the left channel ground and the touch the solder area where the yellow and black wire is. You should get zero's reading on the meter, which means that the wires are good. If you get no reading or a high reading like 003,004 or higher there could be a problem with the wires. You can check all the wires from the end of the tonearm to that board just to make sure everything is okay. You can also use the meter to check the cartridge, again set the meter 2000 ohms the use the probes and touch the white and black/blue pins (left channel), and the red and green pins (right channel). You should get a reading on the meter and both channels should be the same or close. The only way to track down a hum issue is to use a meter otherwise you will just be chasing it all day. I hope this helps in some way.